The present invention relates to an automatic slack adjuster for cam-actuated brakes of an automotive vehicle, and more particularly to a mechanism contained within an adjuster for rotation of the cam acting to expand the brake shoes of an automotive brake against the brake drum wherein the mechanism automatically takes up the play which develops between the movable elements of the brake and the drum as the brake lining and the mechanical parts cooperating therewith undergo normal wear.
Known types of slack adjusters for the brakes of heavy duty vehicles have utilized a manual setting arrangement for the adjuster, making it necessary to check and readjust each brake shoe slack adjuster periodically. To do this, brakes are usually snugged up to create a heavy drag and then they are loosened for a half or more turns of an adjusting screw. The adjustment is then set where the mechanic thinks he has a free wheel, thereby providing a fairly short brake chamber an piston rod travel. As repeated brake applications are made, lining wear occurs, thus gradually increasing piston rod travel to the point where another manual adjustment is required. Therefore, vehicles utilizing the manually set slack adjusters are required to be returned to the shop for adjusting the brakes at short intervals. One of many disadvantages of manual brake adjustment is the likelihood, on the part of the mechanic, to make adjustments on the tight side rather than on the loose side to avoid the usual complaint in a short time that brakes are still too loose. Obviously, this increases the wear on the lining, excessive drag and heat, and creates a much shorter brake life.
To obviate the disadvantages of a manually set slack adjuster, more recent slack adjusters have a feature for taking up the wear on each individual wheel brake by automatic means. However, most known adjusters for camactuated brake systems have been subject to certain limitations. Such adjusters for cam-actuated brakes have provided an adjustment of the cam shaft and cam in response to the total angular rotation of the actuating cam. It is known that a portion of the cam rotation is utilized to move the brake shoes from a non-braking position where a predetermined running clearance exists between the brake shoes and the brake drums to a braking position where the brake shoes come into contact with the brake drum. A further portion of the total cam rotation occurs after initial contact is made by the brake shoes with the brake drum, which further rotation of the cam results from a distortion or deformation of the brake drum, brake shoes, cam shaft and air chamber bracket.
Most previously known automatic adjusters have been unable to distinguish between the brake shoe movement from the position of normal running clearance to a position of initial contact with the brake drums and the further rotation of the cam which results in deformation or distortion of the brake assembly. Also, because of such deformation, prior known adjusters having unlimited incremental adjustment during a single brake application will provide too much adjustment as a consequence of a single very hard application of the brakes, although no adjustment is really required; so that as a result, slack will become too small. The danger of such false adjustment is particularly great where a hand brake is connected to a power operated brake system. The present invention overcomes these above recited limitations to provide an improved automatic slack adjuster system.
Among the objects of the present invention is the provision of a novel automatic slack adjuster for a cam-actuated vehicle brake wherein a unitary rotatable lever and body member is mounted on the cam shaft and is actuated by the power source through a piston rod connected adjacent to the outer end of the lower. The slack adjustment is achieved through a worm gear secured to the cam shaft and a worm operatively connected to the worm gear and adapted to be rotated by a rotor upon excess rotation of the lever. The rotor is rotated by engagement with a plunger mounted in the body member at right angles to the worm shaft and normally spaced from the clevis of the piston rod connected to the lever. A predetermined initial spacing is maintained between the clevis of the piston actuating member for the lever and the upper end of the plunger which corresponds to the normal running clearance between the brake shoes and the brake drum in the non-braking position. Friction means acting on the worm shaft prevents unwanted rotation of the worm shaft when the brake shoes are not engaged with the drum.
Another object of the present invention is the provision of a novel automatic slack adjuster for vehicle brakes utilizing a spring-loaded plunger retractable within itself which activates the rotor on the worm shaft. The plunger provides a limit stop means to hold the rotor in one direction in a pretorqued condition under the force of a torsion spring having one end connected within the lever body and the opposite end connected to the rotor. A one-way spring clutch is provided between the rotor and the worm shaft such that the clutch does not engage in one direction of rotation of the rotor but engages both the rotor and worm shaft in the opposite direction of rotation to provide the adjustment upon wear of the brakes. Second limit stop means limits rotation of the rotor in the opposite direction so adjustments occur in small increments and over-adjustment is avoided.
A further object of the present invention is the provision of an automatic slack adjuster for vehicle brakes utilizing a return spring of the compression type acting to pretorque the rotor in combination with the actuating plunger mounted in the body member at right angles to the worm shaft. A two-part plunger is provided with a compression spring preloaded between the telescoping parts for the downward movement of the plunger to actuate and rotate the rotor and the limit stop means includes a two-part member having a compression spring therebetween to urge the rotor in the opposite direction of rotation against the movement of the plunger and when moved downward the two parts of the limit stop means will abut to limit the total rotation of the actuator in one direction and thus prevent over-adjustment when wear or drum expansion occurs. A limit stop is also provided in the plunger structure to cooperate with the compression spring to preload the rotor in the retracted or non-braking position.
Further objects are to provide a construction of maximum simplicity, efficiency, economy, and ease of assembly and operation, and such further objects, advantages, and capabilities as will later more fully appear and are inherently possessed thereby.